Acoustic transducer arrays, and in particular ultrasonic transducer arrays may be arranged in a number of configurations including linear, one-dimensional arrays, matrix two dimensional arrays, annular ring arrays, etc. While for one-dimensional arrays, techniques such as that described in U.S. Pat. No. 4,404,489, issued to Larson et al on Sep. 13, 1983 and assigned to the assignee of the current application, may be utilized for connecting leads to the transducer, such techniques are not at all suitable for two-dimensional arrays. In particular, referring to FIG. 1 which illustrates a common prior art technique, a linear array 15 of spaced transducer elements 13 is shown, each of which is connected on its bottom surface 17 to a conductive lead 18. Leads 18 may be individual leads which are conductively bonded to a conductive contact area on surface 17, but are preferably printed circuit leads suitably ohmically contacting the element contact areas. Undersides 17 are secured to a backing 22 which provides structural support for the array and which also may provide impedance matching and acoustic damping for reasons to be discussed later. Leads 18 are connected to plated through holes 20 or to contacts on circuit board or flexible cable 19 by wave solder, pressure or other suitable means. Output conductive leads or traces 11 on a printed circuit board 19 extend from each hole/contact 20.
Typically, with a piezoelectric element 13, acoustic waves are transmitted both from the front face 21 of the element and from the rear face 17 thereof. One or more impedance matching layers are generally provided on face 21 to enhance the passage of ultrasonic signals from this face into a body being scanned and to minimize reflections from the element/body interface.
However, the situation at rear face or surface 17 is more complicated. If there is an impedance mismatch at this surface (i.e., if the acoustic impedance of the piezoelectric crystal element 13 is substantially different from the acoustic impedance of backing 22 to which it is attached), then there will be acoustic reflections within the element at surface 17. This improves the power output from the transducer element in the desired direction, but may also result in a wider acoustic output pulse and thus in poor ultrasonic image resolution. This pulse widening may in some applications be overcome by proper selection of impedance matching layers at surface 21.
Further, acoustic signals which do pass through surface 17 may, if not attenuated, reflect off of circuit board 19 and return to the transducer. These reflected signals may cause a degrading of the display in various ways.
It is, therefore, desirable that a mechanism be provided for controlling or eliminating the reflections at surfaces 17 of the transducer elements to achieve a desired balance between output power and image sharpness, and that acoustic signals exiting surfaces 17 be substantially attenuated so that image degrading reflections of such signals are not returned to the transducer element. Backing 22 may, in addition to providing structural support, also be constructed to perform these functions.
However, the approach shown in FIG. 1 is adapted for use only with one-dimensional arrays. An attempt to use the same technique with two dimensional arrays would result in leads 11 and 18 making contact with two or more transducer elements, basically shorting these elements, or when the array is sawed, would result in connection to only the elements around the perimeter of the array. Therefore, it is necessary to provide contact between an electrically conductive area on the underside of each transducer element of a two-dimensional array and a corresponding contact point on a circuit board, strip, semiconductor element (i.e. chip, wafer, layer, etc.) or the like. While techniques exist in the art for effecting such electrical contacts, they are not easily achieved. A way of achieving such contact while still providing the benefits of a backing 22 does not currently exist.
A need, therefore, exists for an improved method and apparatus for making electrical contacts between acoustic transducer arrays in general, and two-dimensional acoustic transducer arrays in particular, and corresponding contacts or traces on an electrical circuit element. Such technique should permit all or a selected portion of the acoustic energy appearing at the rear surface of each transducer element to be outputted from the element rather than being reflected, and for the outputted acoustic energy to be fully attenuated so that there are substantially no reflections of such energy back into the transducer element. Such a technique should also minimize or eliminate acoustic energy entering the transducer leads and/or such acoustic energy as does enter these leads should also be fully attenuated so that such energy results in substantially no reflection back into the transducer. Finally, such technique should also provide solid support for the array.